Return mapping from variables to registers in fate compilation (#411)

* Return mapping from variables to registers

* Fix dialyzer issues

* Record real names

* Report saved fresh names as part of fcode env

* Undo whitespace changes

* Fix dialyzer warnings

* Formatting fix

* Use function names as strings

* Manually handle making function names

* Update CHANGELOG

* Make variables registers optional

* Update docs about the new flag

* Remove empty saved_fresh_names map from fcode env

CHANGELOG.md

@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
     entrypoint spend(x : int) : int = x
     entrypoint f(c : Main) : int = c.spend(10)
   ```
+- Return a mapping from variables to FATE registers in the compilation output.
 ### Changed
 ### Removed
 ### Fixed